1. Field of the Invention
The present invention relates to a solar cell adapted for use as an electric voltage source and an electric power source for various electronic appliances and driving equipment, and a method for producing the same, and more particularly to a solar cell capable of providing a high open-circuit voltage with a small amount of materials by growing single crystals without grain interfaces on small nucleation surfaces formed on a non-nucleation surface, and a method for producing the same.
2. Related Background Art
The solar cell employs, in the functional part thereof, a PN junction of which the semiconductor components are generally composed of silicon. Though monocrystalline silicon is preferred in terms of the efficiency for converting the optical energy into the electromotive force, amorphous silicon is considered advantageous in terms of formation of a large-area device and cost reduction.
In recent years there has been investigated the use of polycrystalline silicon with the conceived advantages of a low cost comparable to that of amorphous silicon and a high energy conversion efficiency comparable to that of monocrystalline silicon. However, in the conventionally proposed methods utilizing a plate-like member sliced from a block of polycrystals, it is difficult to reduce the thickness below 0.3 mm. The thickness is therefore in excess of the value required for sufficient light absorption, and the effective utilization of material is not attained in this respect. Thus a sufficiently thin structure is indispensable for reducing the cost.
For this reason attempts have been made to form a thin polycrystalline silicon film with thin film forming methods such as chemical vapor deposition (CVD), but there have only been obtained crystal grains of the order of several hundredths of a micrometer, and the energy conversion efficiency obtainable with such crystal grains is even lower than that with the sliced block polycrystalline silicon.
Also, there has been reported the so-called "abnormal grain growing" technology, in which atoms of an impurity such as phosphorus are introduced, by ion implantation to a supersaturated state, into a thin polycrystalline silicon film formed by the above-mentioned CVD method, and said film is annealed at a high temperature to increase the crystal grain size to 10 times or more of the film thickness (Yasuo Wada and Sigeru Nishimatsu, Journal of the Electrochemical Society, Solid State Science and Technology, Vol. 125 (1978), pg. 1449), but such crystal grains cannot be used in the active layer for generating photocurrent, because of the excessively high impurity concentration.
It has also been attempted to melt and recrystallize a thin polycrystalline silicon film with laser beam irradiation, thereby increasing the crystal grain size, but the cost reduction is insufficient and stable manufacture is also difficult.
Such situation is not limited to silicon but also commonly exists for compound semiconductors.
On the other hand, a method disclosed in the Japanese Patent Laid-Open Application No. 63-182872 is capable of providing a thin polycrystalline solar cell consisting of a group of single crystals having sufficiently large crystal grain sizes and a satisfactory energy conversion efficiency, by the steps of forming, on a substrate surface, a different material having a nucleation density sufficiently larger than that of the material constituting said substrate and being sufficiently small so as to allow the growth of a single nucleus, forming a nucleus on said different material by deposition, and growing a crystal from said nucleus, thereby forming a monocrystalline semiconductor layer of a first conductivity type on said substrate surface, and then forming a monocrystalline semiconductor layer of a second conductivity type on the above-mentioned semiconductor layer.
However, the above-mentioned conventional method is associated with a drawback that crystal grain boundaries are formed in the positions where the single crystals, grown from small nucleation surfaces composed of said different material, come into mutual contact.
In general, in a polycrystalline semiconductor, many monocrystalline grains with different crystal orientations for many grain boundaries, and defect levels are formed in the forbidden band because atoms with free bonds are present at such grain boundaries. The characteristics of the semiconductor device are closely related to the defect density of the semiconductor layers to be produced. In the polycrystalline semiconductor device, the device characteristics are considered to be significantly affected by the grain boundaries, since such grain boundaries not only contain defect levels but also tend to induce precipitation of impurities, thus leading to deteriorated device characteristics. Stated differently, in order to improve the characteristics of a polycrystalline semiconductor device, it is effective to reduce the amount of grain boundaries present in the semiconductor layers. The above-mentioned method aims at a reduction of the amount of the grain boundaries by an increase in the crystal grain size.
FIGS. 7A and 7B are schematic cross-sectional views of solar cells produced by conventional methods. FIG. 7A illustrates a common polycrystalline semiconductor layer, in which a plurality of grain boundaries 502 are present crossing a junction plane 501. On the other hand, FIG. 7B illustrates a semiconductor consisting of a group of monocrystalline Si bodies formed by the selective crystal growing method, in which crystals are grown on small nucleation surfaces consisting of a different material. In this case, crystal grain boundaries 504 are present between the monocrystalline Si bodies 503.
The PN junction is generally formed in the vicinity of a surface of the semiconductor layer at the light incident side. In the case of a polycrystalline semiconductor, active grain boundaries 502 are included, as shown in FIGS. 7A, in the PN junction, thus generating a current by recombination. Consequently, the dark current becomes significantly higher than in the monocrystalline semiconductor, thereby giving rise to deterioration of characteristics, particularly a decrease in the open-circuit voltage. In an ordinary polycrystalline silicon solar cell, the open-circuit voltage is generally 0.5 V or lower unless a particular treatment such as hydrogen passivation is conducted. Also in the above-mentioned method, in which the second monocrystalline layer is formed as a continuation of the first monocrystalline layer, the grain boundaries are included as shown in FIG. 7B though the amount thereof is less than that in the common polycrystalline semiconductor with small grain sizes, so that the open-circuit voltage is lower than in the monocrystalline semiconductor. In this manner, in the polycrystalline silicon, the recombination current becomes predominant and increases the amount of the dark current. Consequently, if the formation of grain boundaries, giving rise to such recombination current, can be prevented, the dark current will be drastically reduced and the open-circuit voltage is expected to increase significantly.
In a polycrystalline silicon film, prepared for example by ordinary CVD, the crystal grain size fluctuates and the position of the grain boundaries cannot be defined because the nuclei are generated in a random manner. On the other hand, a method disclosed in the Japanese Patent Laid-Open Application No. 63-182872 can define the position of the grain boundaries as the locations of single crystal generation, and the fluctuation in the grain size can be controlled.
FIG. 6A is a view showing the arrangement of nucleation surfaces on a substrate in such silicon film formation; FIG. 6B is a lateral cross-sectional view along a line 6A--6A' in FIG. 6A; FIG. 6C is a view of single crystals grown on said substrate; and FIG. 6D is a cross-sectional view along a line 6B--6B' in FIG. 6C.
On a non-nucleation surface 402 of the substrate 401, there are formed plural nucleation surfaces 404 of a substantially square shape, with a lateral length k and with a pitch l, and selective crystal growth thereon provides a polycrystalline silicon film consisting of silicon single crystals 403 of large grain sizes, with checkerboard-like grain boundaries 405. Such checkerboard-like grain boundaries can be obtained because the positions of generation of nuclei, or of single crystals, and the grain sizes during the growth of single crystals are controlled.
For the purpose of avoiding formation of such grain boundaries, the U.S. Pat. No. 5,094,697 of the present applicant discloses a polycrystalline solar cell consisting of a group of single crystals of satisfactory characteristics without grain boundaries. More specifically, said patent application discloses a photovoltaic element comprising first photovoltaic elements including a plurality of mutually non-contacting semiconductive single crystals formed by a crystal growing process on a substrate having a non-nucleation surface and a plurality of nucleation surfaces which have a nucleation density sufficiently higher than that of said non-nucleation surface and each of which has a surface area sufficiently small for allowing formation of only one nucleus for growing a single crystal, and a second photovoltaic element formed on said substrate so as to cover all of said first photovoltaic elements.
FIG. 8 illustrates, for the purpose of comparison, a solar cell disclosed in the above-mentioned patent of the present applicant, wherein there are shown a stainless steel substrate 1; apertures 2; a SiO.sub.2 layer 3; single crystals 4, 5 respectively of P.sup.+ and P types formed by selective crystal growth; an N.sup.+ -polycrystalline layer 6; an amorphous P-Si layer 7; an amorphous I-Si layer 8; an amorphous N-Si layer 9; and a transparent electrode 10. The illustrated configuration enables a significant increase in the open-circuit voltage, since the single crystals are mutually separated and do not form grain boundaries.
However, such separated configuration significantly increases the surface area of the crystals in comparison with a continuous film of mutually contacting single crystals, and it has been found that an increase in the junction area or in the contact area between the semiconductor layer and the transparent electrode increases the amount of recombination at the interface, thus resulting in a reduction in the open-circuit voltage. The present invention is to provide an improvement for overcoming the drawbacks in such conventional configuration.